public class kehou3_3a {
	public static void main(String args[]) {
		double sum = 0;
		for(int i=1;i<=20;i++) {
			sum = sum+1.0/fact(i);
		}
		System.out.println(sum);
	}
	public static int fact(int n) {
		if(n==0) 
			return 1;
		else
			return n*fact(n-1);
	}
}
